The present invention relates to a liquid crystal display method and a liquid crystal display device which are superior in motion picture display.
Hitherto, there has been available an active matrix type LCD (Liquid Crystal Display) device. In this active matrix type LCD device, as shown in FIG. 31, each time one-horizontal-line data is sampled from an image signal to a sampling memory 2 by a source driver 1, the sampled data is stored into a holding memory 3. On the liquid crystal panel side, a horizontal line made up of a row of pixels into which data is to be written is selected by a gate driver (not shown), and TFTs (Thin Film Transistors) of the selected pixels are turned on. Then, the one-horizontal-line data signal stored in the holding memory 3 is converted from digital to analog form by a D/A converter 4 and written as such via a source line 6 into all the pixels constituting the selected horizontal line.
This operation is executed for all the horizontal lines, by which image writing for one screen is completed. Further, repeating this image writing for one frame enables a wide variety of images to be displayed. Active matrix type LCD devices which perform such display operations have been finding applications for display sections of word processors and notebook personal computers, or for televisions.
In the conventional active matrix type LCD device as described above, since the response speed of liquid crystals, particularly response speed among halftones, is slower than 16.7 ms, which is the one-frame period, there has been a problem of display grade deterioration that afterimages may be seen in motion picture display.
Also, the data signal written into corresponding pixels continues being held while TFTs keep unselected. For this reason, even if the response speed of liquid crystals is increased, there exists an afterimage on the retina due to human eyes' tracing the motion picture. As a result, the display grade deteriorates as another problem.
Under these circumstances, in order to solve the above problems, there has been proposed a liquid crystal display method as shown below (References 1 and 2). In Reference 1, “Japanese Patent Laid-Open Publication HEI 11-109921,” the screen is divided into upper and lower two divisions, and in the first half of the frame period, signal scan for the upper screen is performed while black signal (blanking) scan for the lower screen is performed. In the second half of the frame period, black signal (blanking) scan for the upper screen is performed while signal scan for the lower screen is performed.
In Reference 2, “A New Motion-Picture Compatible LCD Using Pi-Cells, Journal of the Japan Society of Liquid Crystals, 1999, vol. 3, No. 2,” the screen is divided into upper and lower two divisions, and besides the one frame period is divided into time slots corresponding to the number of lines of the whole screen. Then, in the first slot, signal scan for the upper screen is performed while signal scan also for the lower screen is performed simultaneously. In the second slot, black signal (blanking) scan for the upper screen is performed while black signal (blanking) scan also for the lower screen is performed simultaneously. In this way, signal scan and black signal (blanking) scan are iterated sequentially from slot to slot.
According to the above liquid crystal display methods, with regard to one pixel, both image display period and black display period are necessarily included in one frame period, where, in particular, the presence of the black display period makes it possible to achieve image display free from any mixed presence of preceding- and succeeding-frame data. Thus, an improvement in the display performance for motion pictures can be achieved.
However, the liquid crystal display method disclosed in Reference 2 has the following problem. That is, the one frame period is divided into time slots corresponding to the number of lines of the whole screen, and further the screen is divide into upper and lower two divisions. Then, in the first slot, signal scan for the upper screen is performed while signal scan also for the lower screen is performed simultaneously. In the second slot, black signal (blanking) scan for the upper screen is performed while black signal (blanking) scan also for the lower screen is performed simultaneously. In this way, signal scan and black signal (blanking) scan are iterated sequentially from slot to slot. Therefore, when the upper screen starts to be scanned, the lower screen also needs to be scanned simultaneously, making it necessary to once store one-line image data. As a result, the circuitry is complicated, leading to a cost increase, as a problem.
The liquid crystal display method disclosed in Reference 1 has a similar problem. That is, one frame period is divided into first and second halves, and besides the screen is divided into upper and lower two divisions. Then, in the first half of the one frame period, signal scan for the upper screen is performed while black signal (blanking) scan for the lower screen is performed simultaneously. In the second half of the one frame period, black signal (blanking) scan for the upper screen is performed while signal scan for the lower screen is performed simultaneously. In this case, although the storage of image data as in Reference 2 is unnecessary, there still arise disadvantages of complicated circuitry and cost increase due to the screen division.
Needless to say, dividing the screen would give rise to a need for, for example, a double of source drivers, upper and lower, which leads to a cost increase.
Therefore, an object of the present invention is to provide a liquid crystal display method, as well as a liquid crystal display device, which are capable of improving the motion picture display grade by a minimum essential improvement of conventional LCD devices without performing such screen division as in References 1 and 2 and without requiring any special screen storage devices.
In order to achieve the object, there is provided a liquid crystal display method for displaying an image to pixels by supplying a data signal to a plurality of column lines arrayed in parallel to one another and by supplying a select signal to a plurality of row lines arrayed in parallel to one another in a direction in which the row lines intersect the column lines, the pixels to which the image is displayed being made up of liquid crystals located at intersecting points, or vicinities of the intersecting points, between the column lines to which the data signal is supplied and the row lines to which the select signal is supplied, the liquid crystal display method comprising:                a step for supplying the select signal to the nth (where n is a positive integer) row line and also supplying the data signal to the column lines, thereby displaying an image based on the data signal to pixels located at intersecting points between the nth row line and the individual column lines;        a step for next supplying the select signal to the (n+m)th row line, where “m” is a positive integer, and also supplying to the column lines a black display signal for displaying a black image to pixels, thereby displaying the black image to pixels located at intersecting points between the (n+m)th row line and the individual column lines;        a step for iterating the image display operation based on the data signal and the black image display operation while sequentially shifting the row line to which the select signal is supplied; and        a step for, with a return to the first row line if the (n+m)th row line, to which the select signal is supplied, is beyond the last row line, displaying the image based on the data signal and the black image to all the pixels within one frame period.        
With this constitution, unlike References 1 and 2, data signal supply and black display signal supply to the column lines are alternately performed, where the row line to which the select signal is supplied is shifted with n increased as n, n+m, n+1, n+m+1, n+2, n+m+2, . . . , synchronized with the supply of data signal and black display signal. Thus, without dividing the screen or without using a circuit for storing one-screen image data, the data signal is written into all the pixels, and the black display signal is supplied thereto after an elapse of a specified time period corresponding to “m,” and further the state that the black display signal has been written is held until a new image data signal is written for the next frame, by which a black image is displayed. Therefore, in the case where a pixel under white display changes over to black display at the next frame, the black image has already been displayed when the black display signal is written. Thus, there occurs no light leakage of backlight.
Also, an image edge in a motion picture moves at a change of frames and keeps halted during the frame period. However, since the image is felt smoothly moving to humans, there are one period in which the image edge is present forward of the human line of sight and another period in which the image edge is present backward, so that the image edge looks blurred. However, in this invention, since the pixels under image display turn to a black display and disappear before the next data signal is applied, the period in which the image edge is present forward of the human line of sight and another period in which the image edge is present backward resultantly become shorter so that the blur of the image edge is reduced. Thus, the motion picture display grade is improved.
Also, there is provided a liquid crystal display method for displaying an image to pixels by supplying a data signal to a plurality of column lines arrayed in parallel to one another and by supplying a select signal to a plurality of row lines arrayed in parallel to one another in a direction in which the row lines intersect the column lines, the pixels to which the image is displayed being made up of liquid crystals located at intersecting points, or vicinities of the intersecting points, between the column lines to which the data signal is supplied and the row lines to which the select signal is supplied, the liquid crystal display method comprising:                a step for supplying the select signal to the nth row line (where n is a positive integer) and also supplying the data signal to the column lines, thereby displaying an image based on the data signal to pixels located at intersecting points between the nth row line and the individual column lines;        a step for next supplying the select signal simultaneously to a plurality of row lines other than the nth row line, and also supplying to the column lines a black display signal for displaying a black image to pixels, thereby displaying the black image to pixels located at intersecting points between the plurality of row lines and the individual column lines;        a step for iterating the image display operation based on the data signal and the black image display operation while sequentially shifting the row line to which the select signal is supplied; and        a step for, with a return to the first row line if the plurality of row lines, to which the select signal is simultaneously supplied, are beyond the last row line, displaying the image based on the data signal and the black image to all the pixels within one frame period.        
With this constitution, the black display signal is supplied a plurality of times to all the pixels in the second half of one frame period. Accordingly, even if the black display signal supply time is such that enough black image display cannot be achieved only by one-time supply of the black display signal, the black display can be securely achieved by the supply of the black display signal being iterated a plurality of times. Thus, even if the black display signal supply time is insufficient because of a large number of row lines due to high pixel density of the display panel, a high-grade motion picture display free from occurrence of light leakage of the backlight can be achieved.
In one embodiment of the present invention, the plurality of row lines are (n+α·m)th (α=1, 2, . . . , p (where p is a positive integer)) lines.
With this constitution, with regard to one horizontal line, black display is executed iteratively every m-line scans. Thus, the effect of the display contents of the preceding frame on the dielectric characteristics of liquid crystals is eliminated, so that a further higher display grade can be achieved.
In one embodiment of the present invention, the plurality of row lines are (n+α·m)th to (n+α·m+k−1)th (α=1, 2, . . . , p (where p and k are positive integers)) lines.
With this constitution, with regard to one horizontal line, black display is executed iteratively k times every m-line scans. Thus, the effect of the display contents of the preceding frame is further eliminated.
In one embodiment of the present invention, supply time of the data signal and supply time of the black display signal are equal to each other.
With this constitution, since the supply time of the data signal and the supply time of the black display signal are equal to each other, the supply of the data signal and the supply of the black display signal are switched over by a very simple switching control process.
In one embodiment of the present invention, supply time of the data signal is longer than supply time of the black display signal.
With this constitution, the liquid crystal display method is ready also for such cases where enough data signal supply time cannot be taken because of a large number of row lines due to high pixel density of the display panel.
In one embodiment of the present invention, value of the m is set so as to satisfy the following relationship:f×m/N>t where                N is the number of row lines,        f is the one frame period, and        t is response time of liquid crystals at a switch from white display to black display.        
With this constitution, the black display signal supply time in one frame period is set to a time period longer than the response time of liquid crystals which results when white display is switched to black display. Thus, even in pixels on which a white image is displayed based on the data signal, black display is securely executed before the next data signal is applied.
In one embodiment of the present invention, value of the k is set so as to satisfy the following relationship:T×k≧T0 Where                T is one-time supply time of the black display signal, and        T0 is the shortest time of the black display signal that allows white display to be completely changed over to black display.        
With this constitution, the supply time of the black display signal in one frame period is set to a time period longer than the shortest time that allows white display to be switched to black display by k-time supply of the black display signal. Thus, in the case where the black display signal is supplied iteratively k times because of an insufficient supply time of the black display signal, black display is securely executed before the next data signal is applied, even in pixels on which a white image is displayed based on the data signal.
In one embodiment of the present invention, a voltage Vd for a case where the data signal is a data signal for black display and a voltage Vr of the black display signal are set so as to satisfy the following relationship:                for positive polarity with respect to a potential level of a counter electrode,        Vd<Vr in normally white mode, and        Vd>Vr in normally black mode; and        for negative polarity to the potential level of the counter electrode,        Vd>Vr in the normally white mode, and        Vd<Vr in the normal black mode.        
With this constitution, even when enough black display cannot be executed because of an insufficient supply time of the black display signal, black display is securely executable by preparatorily setting the voltage for the black display signal to a somewhat larger (small) one.
Also, there is provided a liquid crystal display device having: a display panel in which are formed at least a plurality of column lines arrayed in parallel to one another, a plurality of row lines arrayed in parallel to one another in a direction in which the row lines intersect the column lines, and pixels made up of liquid crystals located at intersecting points, or vicinities of the intersecting points, between the column lines and the row lines; a column line driver for supplying a data signal to the column lines; and a row line driver for supplying a select signal to the row lines, the liquid crystal display device comprising:                a display control section for supplying an image signal and a control signal to the column line driver, while supplying a control signal to the row line driver, thereby controlling image display operation to the display panel;        black display signal generating means for generating a black display signal to thereby display a black image to the pixels; and        a selector switch provided in the column line driver and operative for switchedly selecting alternately between a data signal based on an image signal derived from the display control section and a black display signal derived from the black display signal generating means, wherein        the display control section supplies to the row line driver the control signal for making the row lines sequentially selected, where the select signal is supplied to the nth row line while the data signal is selected by the selector switch, and where the select signal is supplied to the (n+m) row line while the black display signal is selected by the selector switch.        
With this constitution, based on the control signal from the display control section, the row line driver and the column line driver are controlled as follows. When the data signal is selected by the selector switch for the column line driver and supplied to column lines, the nth row line is selected by the row line driver. Meanwhile, when the black display signal is selected by the selector switch and supplied to column lines, the (n+m)th row line is selected. Thus, the data signal is written into all the pixels, and after an elapse of a specified time period corresponding to “m,” the black display signal is supplied, and further the state that the black display signal has been written is held until a new image data signal is written for the next frame, by which a black image is displayed. Therefore, in the case where a pixel under white display changes over to black display at the next frame, the black image has already been displayed when the black display signal is written. Thus, there occurs no light leakage of backlight.
Also, there is provided a liquid crystal display device having: a display panel in which are formed at least a plurality of column lines arrayed in parallel to one another, a plurality of row lines arrayed in parallel to one another in a direction in which the row lines intersect the column lines, and pixels made up of liquid crystals located at intersecting points, or vicinities of the intersecting points, between the column lines and the row lines; a column line driver for supplying a data signal to the column lines; and a row line driver for supplying a select signal to the row lines, the liquid crystal display device comprising:                a display control section for supplying an image signal and a control signal to the column line driver, while supplying a control signal to the row line driver, thereby controlling image display operation to the display panel;        black display signal generating means for generating a black display signal to thereby display a black image to the pixels; and        a selector switch provided in the column line driver and operative for switchedly selecting alternately between a data signal based on an image signal derived from the display control section and a black display signal derived from the black display signal generating means, wherein        the display control section supplies to the row line driver the control signal for making the row lines sequentially selected, where the select signal is supplied to the nth row line while the data signal is selected by the selector switch, and where the select signal is supplied to a plurality of row lines other than the nth row line while the black display signal is selected by the selector switch.        
With this constitution, based on the control signal from the display control section, the row line driver and the column line driver are controlled as follows. When the data signal is selected by the selector switch for the column line driver and supplied to column lines, the nth row line is selected by the row line driver. Meanwhile, when the black display signal is selected by the selector switch and supplied to column lines, a plurality of row lines other than the nth line are selected. Accordingly, even if the black display signal supply time is such that enough black image display cannot be achieved only by one-time supply of the black display signal, the black display can be securely achieved by the supply of the black display signal being iterated a plurality of times. Thus, even if the black display signal supply time is insufficient because of a large number of row lines due to high pixel density of the display panel, a high-grade motion picture display free from occurrence of light leakage of the backlight can be achieved.
In one embodiment of the present invention, the row lines are divided into L (where L is a positive integer) blocks on an m-line basis;                the row line driver comprises L partial row line drivers for supplying a select signal to row lines of each block.        
With this constitution, when the data signal is supplied to a column line by the selector switch, the nth row line connected to one partial row line driver is selected by the one partial row line driver. Meanwhile, when the black display signal is supplied to a column line by the selector switch, the nth row line connected to one partial row line driver located at the just rear column of the partial row line driver is selected by the partial row line driver. Thus, the selection operation of (n+m) row lines can be achieved by simple control.
In one embodiment of the present invention, the control signal from the display control section to the column line driver includes a switching control signal for controlling switching operation performed by the selector switch; and                the switching control signal makes select time of the data signal longer than select time of the black display signal.        
With this constitution, the supply time of the data signal is longer than the supply time of the black display signal. Therefore, the liquid crystal display device is ready also for such cases where enough data signal supply time cannot be taken because of a large number of row lines due to high pixel density of the display panel.
In one embodiment of the present invention, the control signal from the display control section to the column line driver includes a switching control signal for controlling switching operation performed by the selector switch; and                the switching control signal makes select time of the data signal and select time of the black display signal equal to each other.        
With this constitution, since the supply time of the data signal and the supply time of the black display signal are equal to each other, the supply of the data signal and the supply of the black display signal are changed over by a very simple switching control process.
In one embodiment of the present invention, the control signal from the display control section to the row line driver includes a discriminant signal for discriminating whether it is a black display signal supply period during which the black display signal is supplied; and                based on the discriminant signal, the row line driver supplies the select signal to the (n+m)th to (n+m+k−1)th row lines during the black display signal supply period.        
With this constitution, the black display signal is supplied to all the pixels k times during a specified time period corresponding to “m” before the next data signal is applied. Accordingly, even if the black display signal supply time corresponding to the “m” is insufficient to fulfill black image display, the black display can be securely achieved by the supply of the black display signal being iterated k times. Thus, even if enough black display signal supply time cannot be taken because of a large number of row lines due to high pixel density of the display panel, a high-grade motion picture display free from occurrence of light leakage of the backlight can be achieved.
In one embodiment of the present invention, the control signal from the display control section to the row line driver includes a scan start signal, and wherein                the row line driver comprise:        a shift register having a plurality of latch circuits; and        scan start signal supplying means for supplying the scan start signal to the first latch circuit of the shift register during a data signal supply period, and also supplying the scan start signal to continuous k latch circuits starting from the mth latch circuit of the shift register during a black display signal supply period.        
With this constitution, a row line driver capable of supplying the black display signal k times before the next data signal is applied can be realized with a simple construction that the row line driver having a shift register is equipped with the scan start signal supplying means.
In one embodiment of the present invention, the scan start signal supplying means is enabled to change the latch circuit number “m” and the number of latch circuits “k” for the black display signal supply period.
With this constitution, by the latch circuit number “m” being changed, the time at which the black image is displayed is changed before the next data signal is applied. Besides, by the latch circuit count “k” being changed, the number of times the black display signal is supplied is changed before the next data signal is applied.
In one embodiment of the present invention, a liquid crystal display device further comprises:                supply control means for controlling operation of the scan start signal supplying means, and        the supply control means outputs a control signal for setting the latch circuit number “m” to the scan start signal supplying means based on a scan-start-position designating signal from external.        
With this constitution, based on the signal from external, the time at which the black image is displayed is changed before the next data signal is applied.
In one embodiment of the present invention, the display control section, in response to a command signal from external, selectively outputs a control signal for a first display mode in which a black display signal supply operation based on an operation performed by the selector switch is performed, or a control signal for a second display mode in which a black display signal supply operation is not performed with the selector switch out of operation.
With this constitution, the display mode is switched between the first display mode that involves increased energy consumption because the black display signal is supplied to column lines and the second display mode that involves less energy consumption based on operation of the selector switch frame by frame. Thus, waste of energy resulting when the display mode is normally fixed to the first mode is prevented.
In one embodiment of the present invention, a liquid crystal display device further comprises:                a signal-use reference power supply for setting a voltage of a data signal supplied from the column line driver, wherein        the voltage of the signal-use reference power supply is changeable between the first display mode and the second display mode.        
With this constitution, in the first display mode in which the black display signal is supplied after the write of the data signal and the black image is displayed before the write of a new image data signal for the next frame so that the transmissivity of liquid crystals is lowered, the voltage of the signal-use reference power supply is changed over so that the voltage of the data signal is set in response to the lowering of the liquid-crystal transmissivity. Thus, a constant gray-level balance is maintained between the first display mode and the second display mode.
In one embodiment of the present invention, a liquid crystal display device further comprises:                motion picture/still picture discriminating means for monitoring data of the same position on a screen based on an image signal derived from the display control section, thereby discriminating whether a picture based on the image signal is a motion picture or a still picture, and outputting the command signal representing a result of the discrimination to the display control section.        
With this constitution, whether or not a picture is a motion picture or a still picture is discriminated based on the image signal by the motion picture/still picture discriminating means, and a command signal representing the discrimination result is outputted to the display control section. Thus, for a motion picture display, in which the display grade is liable to deteriorate, a control signal for the first display mode is automatically outputted from the display control section, and the black image is displayed after the write of the data signal during one frame period and before the application of the data signal for the next frame. Thus, the display grade is improved.
In one embodiment of the present invention, a liquid crystal display device further comprises:                a backlight for illuminating the display panel from its rear side; and        backlight adjusting means for switching brightness of the backlight between the first display mode and the second display mode according to the command signal.        
With this constitution, during one frame period, in the first display mode in which the black image is displayed after the write of the data signal and until the application of the data signal for the next frame so that the transmissivity of liquid crystals is lowered, the brightness of the backlight is increased by the backlight adjusting means. In the normal second display mode, the brightness of the backlight declines. Thus, waste of energy resulting when the brightness of the backlight is kept normally higher is prevented.
In one embodiment of the present invention, the black display signal generating means is a black display signal use power supply, and                voltage of the black display signal power supply is changeable between the first display mode and the second display mode.        
With this constitution, during one frame period, in the first display mode in which the black image is displayed after the write of the data signal and until the application of the data signal for the next frame, the voltage of the black display signal power supply is changed over so that black display is securely achieved.